The aromatic plant Melissa officinalis and effects of its aqueous extracts on summer annual and invasive weed species

The effects of aqueous extracts of the aromatic plant Melissa officinalis were studied on the seed germination and early seedlings growth of ten summer annual species in Petri dish bioassays and pot experiments. The in vitro experiments on the aqueous extracts from M. officinalis on seed germination shows that the extracts of 5 and 10% were the most active inhibitors for all the studied weed species. Seed germination reduction by the aqueous extracts was up to 54% of the untreated seed lot for each species. On the contrary, the concentration of 1% resulted in germination reduction ranging from 1 to 11%, while in some of the weed species (P. minor, S. nigrum, P. angulata and C. albida) the effect was rather stimulatory. This finding is in full agreement with "novel weapons hypothesis" and supports that native plants compared with invasive (like C. albida and P. angulata) are affected more due to the absence of tolerance or resistance to the allelochemicals. The allelopathy RI of aqueous extracts of lemon balm was negative in most cases, while in the case of 10% concentration, emergence was reduced by 58, 54, 48, 46 and 43% for X. strumarium, C. album, S. faberi, C. canadensis and C. bonariensis, respectively. The allelopathic activity of M. officinalis could be exploited in future studies, to identify and isolate the allelochemicals, as models for future herbicides for integrated weed management.